Without this header, we get a compiler error in some configurations:

.../dc/dcn20/dcn20_hwseq.c: In function 'dcn20_hwss_wait_for_blank_complete':
.../dc/dcn20/dcn20_hwseq.c:1493:3: error: implicit declaration of function 'udelay' [-Werror=implicit-function-declaration]

Note: the use of udelay itself may be problematic, as can occupy
the CPU for 200ms in a busy-loop here.

Fixes: 7ed4e635 ("drm/amd/display: Add DCN2 HW Sequencer and Resource")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>